1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Correct formula to use for potential difference?

  1. Sep 14, 2013 #1
    My book can't stick to one formula EVER.
    In Q7 They start off using: E = −ΔV Δs (equation 20-4)

    Then magically go to: ΔV = EΔs

    Where is the negative sign because it never states to find the magnitude of the potential difference

    http://img198.imageshack.us/img198/6914/msbn.jpg [Broken]

    I don't see how ΔV is directionless because in problem 3 it takes on the negative sign

    http://img90.imageshack.us/img90/6047/pa9.gif [Broken]
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Sep 14, 2013 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    You copied one of the equations wrong: you wrote " E = - ΔV Δs. "

    Here's the story: (vectors in bold)

    the correct formula is V2 - V1 = ΔV = - Es where Δs is defined positive pointing from point 1 to point 2
    and E is the force on a 1 Coulomb charge anywhere between points 1 and 2.

    BUT: the 1 coulomb charge is assumed to not affect the given E field. So, more correctly,
    E = limit as q → 0 of (force on a positive charge q)/q.

    Example: capacitor plates with positive voltage on plate 2 and negative voltage on plate 1.
    s points from plate 1 to plate 2.
    ΔV = V2 - V1 = -Es > 0.
    where Δs is positive since we're going from 1 to 2.
    So E = - ΔV/Δs which would be negative. The force on a small positive test charge q is always qE so the force is negative and points in the direction opposite to Δs.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted